Communication systems, such as Code Division Multiple Access (CDMA) systems, facilitate communication between mobile units and landline units. A mobile unit communicates with a Base Transceiver Station, which in turn is connected to a Base Station Controller (BSC) The BSC is connected to a Mobile Switching Center (MSC), which in turn can be coupled with a Public Switched Telephone Network (PSTN). The PSTN provides communication with landline units.
FIG. 1 depicts a block diagram of current three-party conference architecture of a communication system in accordance with the prior art. In a three-party communication including two mobile units 101 and 102 and a fixed unit 103, the decoded message from BSC 105 is sent to a three-party conference circuit 107. The three-party conference circuit 107 includes a plurality of combiners 109 and 110 for combining the messages for output to the users. For instance, the decoded message sent from the originating BSC 105 is sent to a first combiner 109 that outputs to the fixed unit 103 and a second combiner 110 that outputs to the destination mobile unit 102.
In a typical mobile-to-land digital communication call, speech is encoded at the mobile unit prior to transmission to the BSC. The BSC includes a decoder for decoding the mobile encoded message. The decoded speech is then passed to the fixed unit.
For digital mobile unit-to-mobile unit calls, voice is typically encoded prior to transmission by the mobile unit. The encoding is done by speech compression algorithms that are well known in the art. This encoded voice data is typically decoded by the BSC. The decoded voice data is then transmitted to the BSC associated with the destination mobile unit. The decoded voice is encoded by the BSC prior to transmission to the destination mobile unit. This encoding is typically done using the same speech compression algorithms as the transmitting mobile unit. The destination mobile unit receives the encoded voice data, and decodes the voice at a decoder in the mobile unit. The decoded voice is then audibly emitted for the user to hear.
However, voice quality can be severely degraded due to the speech having to pass through multiple speech compression algorithms placed in series. Further, there is increased delay introduced into the system due to the multiple encoding/decoding steps. To alleviate these degradations, a technique called vocoder bypass may be deployed. Vocoder bypass essentially bypasses the decoder in the first BSC and the encoder in the second BSC. The mobile-encoded packets are therefore essentially passed to the other mobile unit without being encoded and decoded.
However, when a third party is added to a call, vocoder bypass cannot be utilized. This is due to the need to sum speech signals together. This summing is not possible with speech encoded packets. Hence when the third party is added to the call, the system must disable vocoder bypass and return to the multiple compression steps, thereby incurring increased voice quality degradation. An additional problem is that the voice quality may be degraded by the speech encoders receiving multiple inputs, since the compression algorithms used in the encoders typically are modeled using a single voice and are often inadequate for multiple speech inputs. In addition, there can be gaps in the audio when switching from vocoder bypass to three-party conferencing.
Consequently, a need exists for a communication system and method for processing three-party conference calls that alleviates the problems associated with the prior art while maintaining voice quality.